1. Field of the Invention
This invention relates generally to semiconductor fabrication processes, and to processes for fabricating polycrystalline silicon resistors. More particularly, the invention relates to a method of fabricating polycrystalline silicon resistors having temperature coefficients which are nearly zero or even positive.
2. Description of the Prior Art
Semiconductor structures and integrated circuits are manufactured using a wide variety of well known techniques. In the manufacture of semiconductor devices or integrated circuits, active and passive components are formed, typically on a silicon substrate, and then interconnected in a desired manner.
Resistors in such structures typically are formed using one of two techniques. According to a first technique, regions of the semiconductor silicon substrate itself are doped with an impurity such as boron, phosphorus or arsenic, which impurity renders the doped regions conductive, but with a desired resistivity. By forming ohmic connections to a pair of spaced-apart locations in such regions, a "diffused" resistor is provided.
A second technique for fabricating resistors in semiconductor structures is to deposit a layer of polycrystalline silicon over the substrate, but separated therefrom by an insulating layer, and then lightly dope the polycrystalline silicon with a desired impurity to render it conductive to the desired extent. To complete the resistor, ohmic connections are formed to a pair of spaced apart regions on the polycrystalline silicon. Compared to diffused resistors, polysilicon resistors offer a significant advantage because the polycrystalline layer does not consume any area in the silicon or other semiconductor substrate. Thus, the silicon remains available for the fabrication of active components, while resistors interconnecting components may be formed directly above the components themselves. Additionally, because of the insulating layer which separates the resistors from the substrate, the polycrystalline silicon resistors have a substantially lower capacitance with the substrate than do diffused resistors.
Unfortunately, these advantages of polycrystalline silicon resistors are often offset by the undesirable negative temperature coefficient of such resistors. A negative temperature coefficient means that as temperature increases, the resistance of such resistors decreases. The negative temperature coefficient is undesirable because it may result in circuits which are thermally unstable. That is, because the resistors in the circuit conduct more current at higher temperatures, more current flows through the circuit and the circuit dissipates more power. The increased power dissipation results in a further rise in temperature, a further decline in resistance, which in turn causes additional current, higher power, and a still higher temperature. Because of this undesirable thermal characteristic, polycrystalline silicon resistors have not been employed in many circuits where their application otherwise would be ideal.
It is known that polycrystalline silicon resistors having a positive temperature coefficient may be fabricated by employing massive doses of impurity. Unfortunately, such massive doses greatly lower the resistance per unit area. As a result, to obtain high resistances, huge resistors are required. Such large resistors are undesirable, particularly as continuing advances in semiconductor fabrication technology result in the remainder of the circuit being smaller and smaller.